D8 Series User’s Guide
Chapter 5: Tuning and Control
88
Heat Off
Heat On
Set Point
Set Point - Hysteresis
Process
On
Heat Off
Output
Off
Variable
980
˚
F
1000
˚
F
Figure 5.1
– On/Off Control
Proportional Control (P)
Proportional control eliminates cycling by increasing or decreasing the output proportionally with the
process variable’s deviation from the set point.
The magnitude of proportional response is defined by the proportional band. Outside this band,
the output is either 100 percent or 0 percent. Within the proportional band the output power is
proportional to the process variable’s deviation from the set point.
For example, if the set point is 1000°F and the proportional band is 20°F, the output power is as
follows:
• 0 percent when the process variable is 1000°F or above
• 50 percent when the process variable is 990°F
• 75 percent when the process variable is 985°F
• 100 percent when the process variable is 980°F or below
However, a process that uses only proportional control settles at a point above or below the
set point; it never reaches the set point. This behavior is known as offset or droop. When using
proportional control, configure the manual reset parameter for the power level required to maintain
set point.
Offset
Proportional
Set Point
Process Variable
Band
Figure 5.2
– Proportional Control
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